Enable swift in tempest-slow-py3 job

Swift is ready on py3 since stable/ussuri and we enabled
it in tempest-full-py3 job for stable/ussuri onwards.

This commit does the same for tempest-slow-py3 job also so that
we can run swift slow test in this job.

Change-Id: I7fa41d4272f792d6fd00bece202c44e342dfeb0a
2 files changed